This invention relates to the rejuvenation or reactivation of deactivated automobile emission control catalysts. More particularly, it relates to the rejuvenation of such catalysts by at least partially removing lead and/or phosphorus compounds from the surfaces thereof.
Much research has been conducted to discover a method for reducing the air pollutants in the exhaust gas of automobile engines. At the present time it is thought that the most feasible method involves, or will involve, the utilization of some form of catalytic converter. This device generally operates in two stages, the first to reduce NO.sub.x compounds to nitrogen, and the second to oxidize CO to CO.sub.2 and hydrocarbons to CO.sub.2 and H.sub.2 O.
Several combinations of catalysts are known in the art which can achieve the desired activity and selectivity for NO.sub.x conversion in the first stage and for the oxidation of CO and hydrocarbon gases in the second. Despite their relatively high cost, preferred catalysts comprise rhodium, with or without added nickel, on bases of alumina or aluminum borate for the NO.sub.x reduction stage, and platinum and/or palladium on similar bases for the CO/hydrocarbon oxidation catalyst. When exposed to exhaust gases containing even very minor amounts of compounds of lead and/or phosphorus, these catalysts eventually become deactivated due to contaminant poisoning. This usually occurs after about 20,000 miles of engine operation, or sooner if the engine is not operated exclusively on Federally Certified gasoline (a maximum of 0.05 of Pb and 0.005 g P per gallon).
Research into the nature of the contaminant poisons reveals that the chemical compounds responsible for catalyst deactivation are derived from the compounds of Pb, P and S present in the gasoline, P, S, Zn, Ca, Mg and Ba present in motor oil and Fe, Cu, Cr and Ni present in the metals composing the engine. The catalytic poisons other than compounds of P, S and Pb generally deactivate the catalysts only when present in large amounts, the primary mechanism of their deactivation being by inhibiting the diffusion rate of the exhaust gas to the catalyst surface and by shielding the active catalytic sites. Lead, sulfur and phosphorus, which deposit mainly as the compounds, PbO, PbO.sub.2, nPbO.PbCl.sub.2 , nPbO.PbBr.sub.2, 3Pb.sub.3 (PO.sub.4).sub.2.PbCl.sub.2 (or Br.sub.2), PbO.Pb.sub.3 (PO.sub.4).sub.2, PbSO.sub.4, PbCl.sub.2, PbBr.sub.2, nPbO.PbSO.sub.4 and Pb.sub.3 (PO.sub.4).sub.2 are known rapidly to deactivate the noble metal catalysts when as little as 0.3 to 8 percent by weight, determined as elemental P and/or Pb, of such compounds deposit thereon. A major problem confronting industry lies in developing effective and economical methods for rejuvenating such poisoned catalysts. To date, no such method has been developed.
It has now been found that the contaminant poisons of lead and phosphorus can be removed from NO.sub.x reduction and CO/hydrocarbon oxidation catalysts, thereby effectively regenerating said catalysts, by the three-step process of subjecting such catalysts to (1) an optional extraction of some of the poisons with an an aqueous solution of one or more select ammonium and/or acetate salts, (2) reduction in an atmosphere preferably containing H.sub.2 and/or CO, and (3) extraction once again with an aqueous solution of one or more select ammonium and/or acetate salts. In addition to lead and phosphorus, it has also been found that other deposited contaminant poisons of Fe, Zn, Mg, Ca, etc., are removable by the process of the invention. Catalytic rejuvenation in the manner described herein recovers between 50 and 75% or more of the activity loss attributable to contaminant poisoning.
A somewhat perplexing aspect of the invention resides in the determination of the function of the reduction step. While the function of the extractions of steps (1) and (3) is primarily to dissolve soluble lead salts, the function of the reduction step is largely unknown. As will be shown hereinafter, it does tend to convert those lead components unextractable in step (1) to a form or forms extractable by the same treatment in step (3). It also seems to be of benefit when phosphorus compounds, e.g., Pb.sub.3 (PO.sub.4).sub.2, which are highly stable under most circumstances, are present in amounts which significantly affect the activity of the catalyst. But the exact mechanism by which these two results are achieved is not known.